This study presents a theoretical investigation of encased composite columns under biaxial loading. The EUROCODE 4 theoretical model for generating the uniaxial interaction diagram was developed and used to meet biaxial requirements. The primary objective was to evaluate how internal steel ratios and configurations and concrete compressive strength influence the axial and biaxial flexural performance of the composite column. Ten models were analyzed, and the results were validated against previous experimental and analytical data. The results indicate that internal reinforcement is the primary driver of stiffness. Changing from an I-section to equivalent reinforcing bars or four steel angles significantly improves the y-axis stiffness. In addition, changes in concrete strength have a negligible effect on stiffness. Increasing concrete strength from 35 to 70 MPa only slightly changes the stiffness. A direct relationship between concrete strength and buckling resistance is observed. The interior steel configuration also shows that the distribution of steel within the column section modifies buckling resistance; therefore, no major reduction in the second moment of inertia is observed in any direction. Regarding flexural resistance, models with internal steel (steel sections or bars) placed at the column corners consistently outperform models with centralized sections. Symmetrical models provide uniform flexural strength across all angles of inclination of the neutral axis (0 to 90), unlike unsymmetrical models, which exhibit a weak axis and reduced flexural resistance. Finally, using steel angles at the column corners represents the most versatile model.